Method of adjusting precursor powder for sintered ore, and precursor powder for sintered ore

ABSTRACT

A precursor powder for sintered ore, which offers excellent sintered ore production efficiency, can be adjusted independently of the quality of iron ore, by setting a mixing ratio [(C/F)×100] of a mass (C) of particles having a particle size of 3 mm or more in coke breeze to a mass (F) of particles having a particle size of 3 mm or more in an iron ore raw material in the range of 2% to 3%.

TECHNICAL FIELD

This disclosure relates to a method of adjusting a precursor powder forsintered ore to be used in a blast furnace, and to a precursor powderfor sintered ore produced by the method.

BACKGROUND

For stable and highly efficient operation of a blast furnace, it isimportant to use high-quality sintered ore with excellent propertiessuch as cold strength, reducibility, and anti-reduction-disintegrationproperties. However, such sintered ore has many control requirements tobe met in production, which presents difficulties in improving the yieldand productivity of products.

Sintered ore is generally produced as follows.

First, coke which is a condensation material, a CaO-containing auxiliaryraw material such as limestone, a SiO₂-containing auxiliary raw materialsuch as nickel slag, and the like are added to and mixed with iron orehaving particles with a particle size of about 10 mm or less, and themixture is mixed and granulated in a drum mixer or the like with theaddition of a proper amount of water. Thereafter, the granular rawmaterials for sintered ore thus obtained are charged, along with cokebreeze, to a pallet of a sintering machine and a raw material layer forsintered ore is formed on the pallet. Then, the raw material layer forsintered ore is ignited with solid fuels on the surface layer partthereof. Then, under the influence of air, solid fuels in the rawmaterial layer for sintered ore are sequentially combusted and sinteredto form a sinter cake. The sinter cake is crushed to moreuniformly-sized particles and those particles having a particle sizeabove a certain level are fed to a blast furnace as sintered ore.

That is, sintered ore results from agglomeration of iron ore in responseto the iron ore being fused by reaction with fluxes, or slag componentssuch as CaO and SiO₂.

Recent years have seen a tremendous growth in demand for steelmaterials, particularly, in emerging markets such as in Asia. As thedemand for steel materials grows, there is an increasing need forsintered ore to be used in a blast furnace and for iron ore as the rawmaterial thereof.

The increase in demand for iron ore is presenting a new challenge thathas not been faced before. That is, it is becoming more difficult tofreely choose the quality of iron ore to be supplied. In particular, forexample, more iron ore supplied to the industry exhibits considerablevariations in particle size distribution.

Additionally, as mentioned above, conventional problems of improvingproduct yield, productivity, and the like still remain unsolved. Thismeans that there is an increasing demand for higher sintered oreproduction efficiency, despite large variations in particle sizedistribution of iron ore.

In producing sintered ore, coke breeze contained in a raw material iscombusted with air passing through a raw material layer for sinteredore. This means that the productivity of sintered ore can be determinedby the air flow rate (air permeability) through the raw material layerfor sintered ore. In addition, air permeability is generally dividedinto two categories: air permeability under cold condition beforesintering, which is determined by the particle size of iron ore and thelike; and air permeability under hot condition during and/or aftersintering, which is determined by the size of pores in sinter cake thatare air passages formed by the flow of a melt. The former, which isdetermined by the particle size of iron ore and the like, is susceptibleto the aforementioned variations in the quality of iron ore rawmaterials, which has posed, in particular, a major challenge to recentefforts to improve productivity.

The solutions that have been proposed to date, however, are notnecessarily effective in solving the aforementioned problems.

It could therefore be helpful to provide a method of adjusting aprecursor powder for sintered ore to be used in a blast furnace and aprecursor powder for sintered ore that are excellent in sintered oreproduction efficiency, despite variations in the particle size of ironore raw materials.

SUMMARY

We discovered that for improved sintered ore production efficiency, itis effective to adjust, in a precursor powder for sintered ore, themixing ratio of the mass of particles of a predetermined shape in cokebreeze to the mass of particles of a predetermined shape in an iron oreraw material within a certain range. That is, in particular, airpermeability under cold condition before sintering may be provided bychanging the properties of the coke breeze depending on the quality ofthe iron ore raw material (with variations in particle size), to provideexcellent air permeability (JPU index) in a precursor powder forsintered ore (a raw material for sintered ore after granulation andpseudo-granulation) in a sintering pallet, thereby offering improvedsintered ore production efficiency.

We thus provide:

-   -   [1] A method of adjusting a precursor powder for sintered ore,        comprising:        -   mixing and granulating an iron ore raw material, coke            breeze, and an auxiliary raw material in a drum mixer to            obtain a precursor powder for sintered ore; and        -   charging the precursor powder to a sintering machine where            the precursor powder is sintered to produce sintered ore to            be used in a blast furnace,        -   wherein the mixing and the granulating are performed with a            mixing ratio [(C/F)×100] of a mass (C) of particles having a            particle size of 3 mm or more in the coke breeze to a            mass (F) of particles having a particle size of 3 mm or more            in the iron ore raw material being adjusted in the range of            2% to 3%.    -   [2] The method of adjusting a precursor powder for sintered ore        according to the aspect [1], wherein the mixing ratio        [(C/F)×100] is set in the range of 2.2% to 2.8%.    -   [3] A precursor powder for sintered ore to be used in a blast        furnace, the precursor powder comprising:        -   an iron ore raw material;        -   coke breeze; and        -   an auxiliary raw material,        -   wherein a mixing ratio [(C/F)×100] of a mass (C) of            particles having a particle size of 3 mm or more in the coke            breeze to a mass (F) of particles having a particle size of            3 mm or more in the iron ore raw material is set in the            range of 2% to 3%.    -   [4] The precursor powder for sintered ore according to the        aspect [3], wherein the mixing ratio [(C/F)×100] is set in the        range of 2.2% to 2.8%.

Even if there are variations in the quality (particle size distribution)of iron ore raw materials, it is possible to reliably obtain excellentair permeability (JPU index) in a precursor powder for sintered ore in asintering pallet, thereby effectively improving sintered ore productionefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

Our methods and powders will be further described below with referenceto the accompanying drawing, wherein:

FIG. 1 is a graph showing the relationship between the JPU and themixing ratio [(C/F)×100] of coke breeze to iron ore raw material.

DETAILED DESCRIPTION

Our methods and powders will now be described in detail below.

The method involves: mixing an iron ore raw material, coke breeze, andan auxiliary raw material in a drum mixer to obtain a precursor powderfor sintered ore; and then charging the precursor powder to a sinteringmachine for sintering the precursor powder to thereby produce sinteredore to be used in a blast furnace. In this case, in particular, anappropriate combination of the iron ore raw material and the coke breezewith a particular focus on the respective particle sizes, as describedlater, ensures high productivity at the time of sintering, namely, highair permeability (JPU index, which will be simply referred to as “JPU”)of a precursor powder for sintered ore in a sintering pallet, which isgiven by Equation (1) below. Note that a larger JPU represents betterair permeability; a JPU of about 22 or more is a particularly goodresult in terms of the productivity of producing sintered ore.

(JPU)=[air flow rate (m³/min)/sintering area (m²)]·[layer thickness(mm)/negative pressure (mmAq)]^(0.6)  (1)

where

-   -   “air flow rate” is an air flow rate through a precursor powder        for sintered ore for a certain sintering area;    -   “sintering area” is a loading area of the precursor powder for        which the aforementioned air flow rate is measured;    -   “layer thickness” is a layer thickness of the precursor powder        where the air flow rate is measured; and    -   “negative pressure” is an atmospheric pressure in a wind box        below the precursor powder.        Note that 1 mmAq=9806.38 Pa.

Particle size is measured by a sieve classification method (JIS R6001(1998)).

Note that examples of the iron ore raw material include hematite orefrom South America, magnetite ore from North America, magnetite ore fromSouth America, pisolite ore and Marra Mamba ore from Australia, and thelike.

The mixing ratio [(C/F)×100] of a mass (C) of particles having aparticle size of 3 mm or more in the coke breeze to a mass (F) ofparticles having a particle size of 3 mm or more in the iron ore rawmaterial is 2% to 3%. It should be noted that to determine F, the massof the iron ore raw material is calculated excluding the mass of returnore.

It is believed that a good JPU may be obtained by controlling theaforementioned mixing ratio [(C/F)×100] via the following mechanism.

When the aforementioned mixing ratio is small, i.e., less than 2, theparticle size of the iron ore is considered to be larger than that ofthe coke breeze. Thus, when the particle size of the coke breeze is toosmall, the sintering rate increases, yet a sintering molten zone becomeswider, thereby deteriorating the air permeability under hot condition.On the other hand, when the mixing ratio is large, i.e., more than 3,the particle size of the coke breeze is coarsened so much that formationof pseudoparticles for which the coke breeze serves as nuclear particlesbecomes apparent during the granulation process. Such pseudoparticlesfor which the coke breeze serves as nuclear particles cannot gain properstrength due to low wettability of the coke breeze and tend to collapseduring the handling process before charged to a sintering pallet, withthe result that more refined pseudoparticles are charged to thesintering pallet to deteriorate air permeability.

It is thus apparent that there is an appropriate ratio of the particlesize of the coke breeze to that of the ore, which can be expressed byC/F×100 and is, as mentioned earlier, 2% to 3%. Note that a preferredrange of the aforementioned C/F×100 is 2.2% to 2.8%.

The auxiliary raw material is not particularly limited to aCaO-containing auxiliary raw material such as limestone, aSiO₂-containing auxiliary raw material such as nickel slag, and thelike, and may include other general, well-known auxiliary raw materialsused in precursor powders for sintered ore and inevitably-incorporatedimpurities.

In addition, the mixing ratio thereof is defined so that CaO/SiO₂(=basicity) is around 2.0 in the resulting sintered ore.

The drum mixer may be a normal drum mixer commonly utilized inproduction of a precursor powder for sintered ore such as a drum mixerwith a cylindrical cone.

In addition, the sintering machine is preferably a bottom-suction DwightLloyd type sintering machine. Other well-known sintering machines mayalso be used to produce a precursor powder for sintered ore.

As described above, it is possible to provide a precursor powder forsintered ore to be used in a blast furnace that comprises an iron oreraw material, coke breeze, and an auxiliary raw material and isexcellent in production efficiency.

That is, a precursor powder for sintered ore may be obtained, with amixing ratio [(C/F)×100] of a mass (C) of particles having a particlesize of 3 mm or more in the coke breeze to a mass (F) of particleshaving a particle size of 3 mm or more in the iron ore raw material,excluding return ore, being 2% to 3%, and preferably 2.2% to 2.8%.

No particular limitation is placed on the conditions other than thosespecified above such as the material of the precursor powder, thefacility and its operational conditions use, and the precursor powdermay be produced according to the conventional methods.

EXAMPLES Example 1

Precursor powders for sintered ore were adjusted under the followingconditions. Then, the resulting precursor powders were fully charged toa bottom-suction Dwight Lloyd type sintering machine to produce sinteredore. We examined JPU during sintering of the precursor powders toidentify the effect.

Iron Ore Raw Material

-   -   Basic unit of iron ore raw material: 1100 to 1200 (kg/t−sr)    -   Percentage of particles having a particle size of 3 mm or more        in iron ore raw material: 30% to 40% (of the charged raw        material)

Coke Breeze

-   -   Basic unit of coke breeze: 45 to 50 (kg/t−sr)    -   Percentage of particles having a particle size of 3 mm or more        in coke breeze: 5% to 20% (of the coke breeze)    -   Mixing ratio [(C/F)×100]: 1.2% to 3.5%    -   Auxiliary raw material (limestone): 6% to 10% (of the charged        raw material)

FIG. 1 shows a relationship between the JPU and the mixing ratio[(C/F)×100] of particles having a particle size of 3 mm or more in thecoke breeze to particles having a particle size of 3 mm or more in theiron ore raw material. It can be seen from the FIGURE that eachprecursor powder for sintered ore that was produced with a mixing ratio[(C/F)×100] satisfying our conditions exhibited a good result in termsof JPU, which was determined to be about 22 or more.

In contrast, each precursor powder for sintered ore produced with amixing ratio [(C/F)×100] not satisfying our conditions yielded a poorresult in terms of JPU, which was determined to be about 19 to 21, i.e.,not more than 21, as shown in FIG. 1.

Example 2

An example in which our method was implemented in an actual machine willbe described below.

As usual, an iron ore raw material to be used in a sintering process wassubjected to automatic sampling in a raw material yard, and thenmeasurements were made of the particle size distribution of the obtainedsamples in accordance with the Japanese Industrial Standards, JIS 8706.

For coke breeze, as usual, undersized lump coke, which had been producedin a coke plant, and the purchased anthracite were sent to a sinteringplant, where they were milled to have a suitable particle sizedistribution for operation. The resulting products thus obtained wereused in the sintering process.

The milling was performed in a rod mill, a cage mill, a ball mill, andthe like. Then, samples were collected from the pulverized coke breezeby a sampler provided at a belt conveyor transfer point, and dried in adryer. A Ro-tap type sieve shaker was used to measure the particle sizedistribution of each sample.

The milling conditions for the coke breeze were adjusted to change thepresence ratio of particles having a particle size of 3 mm or more inthe coke breeze depending on the particle size composition of thereceived iron ore, i.e., the presence ratio of particles having aparticle size of 3 mm or more in the iron ore.

Table 1 shows the measurements of JPU and the mixing ratio [(C/F)×100]of particles having a particle size of 3 mm or more in the coke breezeto particles having a particle size of 3 mm or more in the iron ore rawmaterial (ore). Here, let X (kg/t) be coke component, Y (kg/t) be ore Icomponent, and Z (kg/t) be ore II component, and let x (%) be thepercentage of particles having a particle size of 3 mm or more in thecoke component, y (%) be the percentage of particles having a particlesize of 3 mm or more in the ore I component, and z (%) be the percentageof particles having a particle size of 3 mm or more in the ore IIcomponent, then C=X×x, and F=Y×y+Z×z.

TABLE 1 Percentage of Coke Percentage of Ore I Percentage of Ore II CokeOre I Ore II Particles with Particle Particles with Particle Particleswith Particle Component Component Component Size of 3 mm or more Size of3 mm or more Size of 3 mm or more Test No. (kg/t) (kg/t) (kg/t) (%) (%)(%) (C/F) * 100 JPU 1 49.7 678 86 18.2 44.3 34.0 2.75 24.7 2 49.2 683 9620.3 44.3 34.0 2.98 24.0 3 47.4 710 107 22.1 44.3 34.0 2.99 23.5 4 50.0686 80 17.3 43.4 42.0 2.61 25.1 5 45.2 805 37 24.1 43.4 42.0 2.98 23.9 649.9 676 86 16.0 43.4 42.0 2.42 24.5 7 45.9 797 46 23.6 43.4 42.0 2.9723.7 8 46.8 788 0 14.0 38.5 — 2.16 22.5 9 45.8 779 40 20.6 38.5 42.02.98 24.1 10 45.9 688 153 21.5 38.5 42.0 3.00 23.3

It can be seen from Table 1 that those precursor powders for sinteredore that were produced with a mixing ratio [(C/F)×100] satisfying ourconditions exhibited good results in terms of JPU, which was determinedto be about 22 or more.

In contrast, other precursor powders for sintered ore produced with amixing ratio [(C/F)×100] not satisfying our conditions yielded poorresults in terms of JPU, which was determined to be about 19 to 21,i.e., not more than 21, as shown in Table 1.

In addition, when lines capable of classification and milling of ironore are available, the mixing ratio C/F as specified in our method maybe obtained by adjusting the milling conditions for not only cokebreeze, but also for coarse particles in iron ore.

INDUSTRIAL APPLICABILITY

A precursor powder for sintered ore that offers excellent sintered oreproduction efficiency may be obtained. Our methods may also improveproductivity and maintain air permeability in a blast furnace and,consequently, increase sintered ore yield and sintered ore strength,thereby allowing for stable and highly efficient operation of the blastfurnace.

1.-4. (canceled)
 5. A method of adjusting a precursor powder forsintered ore, comprising: mixing and granulating an iron ore rawmaterial, coke breeze, and an auxiliary raw material in a drum mixer toobtain a precursor powder for sintered ore; and charging the precursorpowder to a sintering machine where the precursor powder is sintered toproduce sintered ore to be used in a blast furnace, wherein the mixingand the granulating are performed with a mixing ratio [(C/F)×100] of amass (C) of particles having a particle size of 3 mm or more in the cokebreeze to a mass (F) of particles having a particle size of 3 mm or morein the iron ore raw material being adjusted to 2% to 3%.
 6. The methodaccording to claim 5, wherein the mixing ratio [(C/F)×100] is 2.2% to2.8%.
 7. A precursor powder for sintered ore to be used in a blastfurnace, the precursor powder comprising: an iron ore raw material; cokebreeze; and an auxiliary raw material, wherein a mixing ratio[(C/F)×100] of a mass (C) of particles having a particle size of 3 mm ormore in the coke breeze to a mass (F) of particles having a particlesize of 3 mm or more in the iron ore raw material is 2% to 3%.
 8. Theprecursor powder according to claim 7, wherein the mixing ratio[(C/F)×100] is 2.2% to 2.8%.